Worksheet: pH Calculations – Answers & Explanations
This document provides the answers and step-by-step explanations for typical pH calculation problems. Understanding these concepts is crucial for grasping acid-base chemistry.
Key Formulas for pH and pOH Calculations
At 25°C, the following relationships are fundamental:
- The definition of pH: pH = -log[H+]
- The definition of pOH: pOH = -log[OH–]
- Relationship between pH and pOH: pH + pOH = 14
- Calculating [H+] from pH: [H+] = 10-pH
- Calculating [OH–] from pOH: [OH–] = 10-pOH
- The ion product of water (Kw): [H+][OH–] = Kw = 1.0 x 10-14
Remember: [H+] refers to the molar concentration of hydrogen ions, and [OH–] refers to the molar concentration of hydroxide ions.
| Question 1 |
| Calculate the pH of a solution with a hydrogen ion concentration [H+] of 1.0 x 10-4 M. Is this solution acidic, basic, or neutral? |
Answer & Explanation:Given: [H+] = 1.0 x 10-4 M Formula: pH = -log[H+] Calculation: pH = -log(1.0 x 10-4) pH = -(-4.00) pH = 4.00 Result: The pH of the solution is 4.00. Acidity: Since pH < 7, the solution is acidic. |
| Question 2 |
| What is the hydrogen ion concentration [H+] of a solution with a pH of 8.50? Is this solution acidic, basic, or neutral? |
Answer & Explanation:Given: pH = 8.50 Formula: [H+] = 10-pH Calculation: [H+] = 10-8.50 [H+] = 3.16 x 10-9 M (Ensure your calculator handles scientific notation correctly) Result: The hydrogen ion concentration [H+] is 3.16 x 10-9 M. Acidity: Since pH > 7, the solution is basic. |
| Question 3 |
| Calculate the pOH of a solution with a hydroxide ion concentration [OH–] of 2.5 x 10-3 M. |
Answer & Explanation:Given: [OH–] = 2.5 x 10-3 M Formula: pOH = -log[OH–] Calculation: pOH = -log(2.5 x 10-3) pOH = -(-2.602) pOH = 2.60 (Typically rounded to two decimal places for pOH, similar to pH) Result: The pOH of the solution is 2.60. |
| Question 4 |
| A solution has a pOH of 11.20. Calculate its pH. Is this solution acidic, basic, or neutral? |
Answer & Explanation:Given: pOH = 11.20 Formula: pH + pOH = 14 Calculation: pH = 14 – pOH pH = 14 – 11.20 pH = 2.80 Result: The pH of the solution is 2.80. Acidity: Since pH < 7, the solution is acidic. |
| Question 5 |
| If the hydroxide ion concentration [OH–] in a solution is 4.0 x 10-10 M, what is the pH of the solution? Is it acidic, basic, or neutral? |
Answer & Explanation:This problem requires two steps: first calculate pOH, then pH. Step 1: Calculate pOH Given: [OH–] = 4.0 x 10-10 M Formula: pOH = -log[OH–] Calculation: pOH = -log(4.0 x 10-10) pOH = -(-9.3979) pOH = 9.40 Step 2: Calculate pH Given: pOH = 9.40 Formula: pH + pOH = 14 Calculation: pH = 14 – pOH pH = 14 – 9.40 pH = 4.60 Result: The pH of the solution is 4.60. Acidity: Since pH < 7, the solution is acidic. Alternatively, you could first find [H+] using [H+][OH–] = 1.0 x 10-14, and then calculate pH from [H+]. Both methods yield the same result. |
Summary of pH Scale
Understanding the pH scale is key:
- pH < 7: Acidic Solution (Higher [H+] than [OH–])
- pH = 7: Neutral Solution ([H+] = [OH–])
- pH > 7: Basic (Alkaline) Solution (Lower [H+] than [OH–])
Remember these are guidelines at 25°C. Temperature can affect Kw and thus the neutral pH value.